1. FIELD OF INVENTION:
This invention relates to a novel method and apparatus for steam pressing a mat of compressible material and a binder into a finished product, and has particular application to the steam pressing mats of lignocelluloses material and thermosetting binders into composite products.
2. DESCRIPTION OF THE PRIOR ART:
For composite board manufactures, press time is one of the most important factors in determining the production rate and thus the production efficiency and cost. Normally, the hot pressing operation involved when forming a panel from a mat of lignocellulosic material and a thermosetting resin or binder controls the rate of panel production, and is a function of the thickness of the panel to be formed.
It has been recognized in the art that steam press time for particle board or like products can be drastically reduced by passing pressurized steam through the pressed board, to thereby effect a faster heat transfer within the pressed board for the purpose of causing the binder or resin to set. To date, however, none of these techniques has proven to be practical or effective for the production of wood-based composites with improved dimensional stability at short press times.
For example, K. C. Shen in U.S. Pat. No. 3,891,738 issued June 24, 1975, discloses the passage of pressurized steam through a mat by introducing steam from one platen, and exhausting the steam transmitted through the mat by the other platen. In this press as disclosed by Shen, the mat is located internally of a peripheral wall or framework which is positioned between the two platens. A proper seal between this peripheral wall or framework and platens is critically required in order to form a sealed chamber when the press is closed. Not only does the peripheral wall surrounding the mat reduce the usable platen area, but because the opposed platens are effectively sealed, a real risk of steam explosion exists.
The steam press described in Canadian Patent No. 1,075,140 - Donald W. Nyberg issued Apr. 8, 1980 is somewhat similar to that of Shen, in that a framework surrounds the mat positioned between the platens and hence creates a closed chamber. Unlike Shen, however, the steam apertures and supply conduits are located in one platen only and thus steaming and exhaustion are through this single or common platen. Consequently, this arrangement cannot provide a pressure differential between the exterior mat surfaces, which results in air and water pockets being formed in the pressed board and which can result in improper resin or binder cure in some areas. Further, the Nyberg press, like Shen, requires a perfect seal about the mat and is also susceptible to explosion.
The press as disclosed by Robert L. Geimer in U.S. Pat. 4,393,019 issued July 12, 1983 is an unsealed steam pressure system in the sense that the strong peripheral walls characteristic of the Shen and Nyberg presses are not required. Thus, hot pressing steam injected into the mat is permitted to escape from all of the mat edges. Further, as the steam is injected into both mat surfaces at the same time, air or water pockets, as above discussed, are formed thus preventing uniform cure throughout the formed board. Moreover, since the system is unsealed, and as cautioned by Geimer, the steam pressure employed should not be such that it blows the mat material out of the press. It would also appear that the Geimer type of press is not capable of producing sufficiently high temperatures within a reasonable time frame to achieve curing of certain binders, such as phenol-formaldehyde resin.